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a

Let charge per unit area is $\sigma = \frac{Q}{4\pi R^2}$ Electric Field due to sphere near its surface$ E = \frac{\sigma}{2\epsilon_0}$ Force on its unit area $ F = \sigma \times E = \sigma \times \frac{\sigma}{2\epsilon_0}= \frac{\sigma^2}{2\epsilon_0} = 515.7 N/m^2$